The present disclosure generally relates to the management of thermal gradients in high-intensity discharge (HID) lamps. In particular, the present disclosure generally relates to coatings for ceramic HID lamps which enable the management of thermal gradients so as to achieve enhanced reliability for such lamps in various applications.
Within various industries including the automotive industry, HID lamps are beginning to replace conventional incandescent halogen lights as lights for headlamps. In a typical HID lamp, light is generated by means of an electric discharge that takes place between metal electrodes enclosed within an envelope sealed at both its ends. The main advantages of HID lamps are high lumen output, better efficiency and longer life. In operation, the lamp size is kept small enough for optical coupling purposes. Further, for automotive applications, the lamps are required to meet industry standards of fast starting, by delivering a major portion of steady state lumens shortly after the point at which they are turned on. The small lamp size and fast start requirements result in potential high thermal loading. These limitations can result in shortening the lamp life and also decreasing reliability of the lamp. To improve reliability, quartz which had been typically used in HID lamp envelopes is being replaced with ceramic material, such as polycrystalline alumina (PCA) and yttrium aluminum garnet (YAG). Ceramic arc-tubes can withstand higher temperatures, which results in higher dose vapor pressure enabling increased efficiency, better color, and higher performance and has increased physical strength and resistance to chemical corrosion, which contribute to a longer operating life.
HID lamps attain high operating temperatures because of the heat associated with the high intensity discharge. Discharge lamps typically produce light by ionizing a vapor fill material such as a mixture of rare gases, metal halides and mercury with an electric arc passing between two electrodes. The electrodes and the fill material are sealed within a translucent or transparent discharge vessel which maintains the pressure of the energized fill material and allows the emitted light to pass through it. The fill material, also known as a “dose,” emits a desired spectral energy distribution in response to being excited by the electric arc. For example, halides provide spectral energy distributions that offer a broad choice of light properties, e.g. color temperatures, color renderings, and luminous efficiencies.
However, despite the advances which have been made in development of HID lamps, including the use of light-emitting ceramic envelopes therefor, there continues to be a need to improve both the performance and reliability of such lamps.